Pulses of electromagnetic radiation, typically in the visible or infrared part of the electromagnetic spectrum, find many uses in science and technology. For instance, almost all existing or planned optical communication systems are of the digital type and thus employ pulses of electromagnetic radiation. Other applications of such pulses are, inter alia, in optical radar, optical ranging, optoacoustic spectroscopy, and reaction rate studies.
The prior art knows many techniques for forming optical pulses, such as rotating chopper discs, pulsed lasers, diodes, or flash lamps. However, most older prior art techniques cannot conveniently and inexpensively produce a train of very short pulses, with pulse width of the order of one nanosecond or less.
Recently some methods have been developed that are capable of producing exceedingly short pulses, in the picosecond, and even femtosecond, range. These include pulse compression methods, (see, e.g., C. V. Shank et al, Applied Physics Letters, Vol. 40(9), pp. 761-763) and the solution laser method (L. F. Mollenauer et al, Optics Letters, Vol. 9, pp. 13-15). Such ultrashort pulses are of great scientific interest, since they permit previously unattainable time resolution in a number of scientific experiments. In addition to their scientific usefulness, such short pulses potentially may be useful in very high bit rate optical communication systems.
Such communication systems, which, it is believed, may be able to operate at bit rates of hundreds of gigabits/second, and even as high as a terabit/second, are based on the use of shape-preserving optical pulses, also referred to as optical solitons. See, for instance, U.S. Pat. No. 4,368,543, issued Jan. 11, 1983 to A. Hasegawa; U.S. Pat. No. 4,406,516, issued Sept. 27, 1983 to A. Hasegawa; and Y. Kodama and A. Hasegawa, Optics Letters, Vol. 8(6), pp. 342-344 (1983). The above patents are co-assigned with this.
Such systems, in order to approach the high data transmission rate of which they are capable, require means that can produce narrow optical pulses at a very high rate. This application discloses a relatively simple and inexpensive method for producing such pulses.
As will be discussed below in detail, the inventive method utilizes the modulational instability of continuous wave (cw) radiation in an appropriate optical medium. This instability has previously been used to produce tunable coherent infrared and far infrared electromagnetic radiation. See, U.S. Pat. No. 4,255,017, issued Mar. 10, 1981, to A. Hasegawa, co-assigned with this, and A. Hasegawa and W. F. Brinkman, IEEE Journal of Quantum Electronics, Vol. QE-16(7), pp. 694-697. The prior art method comprises injection of unmodulated cw radiation, the carrier, into single mode optical fiber, the carrier wavelength chosen to lie within the regime of anomalous dispersion of the fiber core material. Due to the combined effect of the anomalous dispersion and the nonlinear Kerr effect, side bands of the carrier are produced; in other words, amplitude modulation of the injected unmodulated carrier wave results. Rectification of the modulated carrier yields an output signal of a frequency proportional to the square root of the power in the carrier wave.